1. Field of the Invention
The present invention relates to compensating transmission buses and more particularly to inductive compensation of capacitive parasitics of transmission buses to raise the effective impedance of the transmission buses.
2. Description of the Related Arts
As computer technologies continue to grow by leaps and bounds, users of the computer technologies have an insatiable desire for faster systems with increased storage and retrieval capabilities. To address these desires of the users, faster and more powerful CPUs (central processing units) and larger hard disk drives and multiple disk drive enclosures are constantly being developed.
An interface called SCSI (small computer system interface) is a common interface used to connect and communicate storage devices with a CPU. The SCSI specification enables diverse peripheral devices and one or more computer systems to work together. SCSI devices connect to a computer using a SCSI bus in a daisy chain configuration as opposed to being connected in a parallel configuration. The SCSI bus comprises a multitude of transmission lines for control and data signals. One reason for the SCSI popularity among computer systems is that SCSI is a device independent interface. Each SCSI device masks the internal operation of a peripheral from other peripherals and the computer, even though the SCSI devices are connected to the same transmission bus. The computer CPU does not need to know much about each peripheral in order for the computer to communicate with the peripheral. SCSI peripherals are referred to as a logical units rather than physical or actual units and are treated as storage abstractions. The ability for a computer system to treat SCSI devices in abstract terms makes SCSI a popular physical layer.
In applications of high performance data rate transfers in multiple disk drive enclosures which include dedicated multiple disk drive storage and server applications for computer systems, the SCSI physical layer has its limitations. The limitations stem from the fact that as a SCSI disk drive component is connected to the SCSI bus, the SCSI disk drive introduces parasitic capacitance to each SCSI transmission line of the SCSI bus that the SCSI disk drive is connected. The parasitic capacitance or capacitive loads lower the effective impedance of the SCSI transmission line. If the lowered effective impedance is not compensated, reflections and termination problems from impedance mismatches will cause transmission errors of signals on the SCSI transmission lines, particularly at high performance data rates. Ideally, the impedance of the transmission lines of the SCSI bus should be maintained at or about 90 ohms. Currently, many of the multiple disk drive enclosures limit the number of disk drives within the enclosures connected to the SCSI bus in order to reduce the capacitive loading on the SCSI bus so that the impedance of the transmission lines of the SCSI bus are maintained around the 90 ohms range.
Therefore, it is desirable to provide an apparatus and methods of operating the same which compensate capacitive loading from disk drives connected to a transmission bus so that the number of disk drives in a multiple disk drive enclosure is increased without suffering reflections and termination problems associated with impedance mismatches.